1. Field of the Invention
This invention relates to a control device for an automatic transmission, and more particularly to a control device that controls supply and drain of oil pressure to and from frictional elements such as a low-and-reverse brake in rearward driving and 1-range driving.
2. Description of the Prior Art
A conventional control device has been disclosed in Japanese Laid-Open Patent Publication No. 2001-12588 which is intended to achieve favorable gear change characteristics by providing accurate control of supply and drain of oil pressure to and from frictional elements such as a low-and-reverse brake which are engaged at forward-driving low speed gear positions and a rearward driving gear position, and to ensure the torque capacity required for keeping the frictional elements engaged after a gear change by switching to a high pressure such as a line pressure or the like.
As shown in FIG. 1 of the above-mentioned publication, this control device enables accurate control by setting the gain of an output pressure Po from a pressure reduction control valve with respect to a solenoid pressure from a solenoid valve to a small value. If the output pressure is lower than the set switching pressure, a switching valve outputs the output pressure directly to the frictional element (i.e. the low-and-reverse brake).
On the other hand, in a circuit shown in FIG. 6 of the above-mentioned publication, if the output pressure Po has become equal to or higher than the set switching pressure, the switching valve outputs a line pressure PL instead of the output pressure Po to the low-and-reverse brake to ensure a large torque capacity required for engagement of the low-and-reverse brake.
Further, in a circuit shown in FIG. 9 of the above-mentioned publication, if a pressing force generated by the line pressure has become larger than a pressing force generated by a spring, the switching valve outputs the line pressure PL to the low-and-reverse brake.
The above-described conventional control device has the disadvantage that it is impossible to freely set the timing for switching the switching valve since the timing for switching the switching valve is uniquely defined by the set spring force.
If an R (rearward) range is selected while a vehicle is running forward, oil pressure must be inhibited from being supplied to the low-and-reverse brake so as to prevent the low-and-reverse brake from being engaged, because the low-and-reverse brake is the frictional element that is engaged at the forward-driving low speed gear positions and the rearward-driving gear position. Further, in the case where a power supply has failed, oil pressure must be supplied to the low-and-reverse brake if the R range is selected, and oil pressure must be inhibited from being supplied to the low-and-reverse brake if a D range is selected. The above-described conventional control device, however, cannot satisfy these requirements.
Specifically, in the circuit shown in FIG. 6 of the above-mentioned publication, if the solenoid valve is a normal-high type solenoid valve (which outputs oil pressure when no current is carried), when the R range is selected in the case where the power supply has failed, oil pressure supplied from the pressure reduction control valve switches the switching valve to supply the line pressure to the low-and-reverse brake. Even if the D range is selected, however, the oil pressure is supplied to the low-and-reverse brake.
Further, although the circuit shown in FIG. 9 of the above-mentioned publication is capable of inhibiting the low-and-reverse brake from being engaged if the R range is selected by mistake while the vehicle is running forward, the range of control is restricted because the line pressure must be kept at a low pressure. Further, irrespective of whether a normal-high type solenoid valve or a normal-low type solenoid valve (which outputs no oil pressure when no current is carried) is used, oil pressure is supplied to the low-and-reverse brake if the R range or the D range is selected in the case where the power supply has failed.
To satisfy all of the above requirements, i.e. to ensure the freedom of setting the timing for switching the switching valve, the conventional control device requires an additional solenoid valve that is used to switch the switching valve. Further, to satisfy the requirements when the R range is selected while the vehicle is running forward and in the case where the power supply has failed, the conventional control device must be provided with an increased number of parts such as an additional switching valve. This makes it impossible to reduce the size of the control device, and increases the cost.
It is therefore an object of the present invention to provide a control device for an automatic transmission, which inhibits a gear shift to a rearward-driving gear position even if an R range is selected while a vehicle is running forward, and enables the vehicle to run rearward even in the case where a power supply has failed, while providing accurate hydraulic control and ensuring the torque capacity required for engagement of a frictional element with only a small number of additional parts.
To attain the above object, the present invention provides a control device for an automatic transmission, comprising: a first solenoid valve capable of outputting a first solenoid pressure; a second solenoid valve capable of outputting a second solenoid pressure; a pressure reduction control valve that outputs an oil pressure controlled to be reduced based on a supplied original pressure if the second solenoid pressure is supplied, and inhibits the original pressure from being outputted if the second solenoid pressure is unsupplied; and a first shift valve that is selectively switched such that an output from the pressure reduction control valve is supplied to a first frictional element engaged at gear positions including a rearward-driving gear position if the first solenoid pressure is supplied, and that an R range pressure which is a line pressure supplied from a manual valve and generated only in a case where an R range is selected is supplied to the first frictional element if the first solenoid pressure is unsupplied.
If the R range is selected, both the first solenoid pressure and the second solenoid pressure are supplied to output an oil pressure reduced by the pressure reduction control valve according to the second solenoid pressure. Since the gain of the pressure reduction control valve can be set to a small value, it is possible to provide accurate control in a gear change or the like. The oil pressure thus reduced is supplied to the first frictional element via the first shift valve to engage the first frictional element to enable a shift to the rearward-driving gear position.
In this state, by stopping the supply of the first solenoid pressure and switching the first shift valve, the R range pressure is supplied instead of the reduced control pressure to the first frictional element to engage the first frictional element. Since the R range pressure is equal to the line pressure, it is possible to ensure the required transmission torque capacity required after the engagement.
If the R range is selected by mistake while the vehicle is running forward, the first solenoid pressure is supplied and the supply of the second solenoid pressure is stopped in response to detection of the R range selection.
This causes the first shift valve to connect the first frictional element with the pressure reduction control valve, but inhibits the pressure reduction control valve from outputting oil pressure. Therefore, the first frictional element is not engaged to inhibit a shift to the rearward-driving gear position.
Further, since neither the first solenoid pressure nor the second solenoid pressure is supplied in the case where the power supply has failed, the selection of the R range causes the R range pressure to be supplied via the first shift valve to the first frictional element to engage the first frictional element and enable a shift to the rearward-driving gear position. On the other hand, the selection of the D range causes the first shift valve to connect the first frictional element with the R range pressure, but the first frictional element is not engaged since no R range pressure is supplied.